Distrust between former allies arose right after the end of WWII. The capitalist Allied forces were unsure on whether they could trust Stalin since he had previously allied himself with Hitler in 1939 through the Nazi-Soviet Pact.
Also, Stalin thought that the US and the British joined the war late because they expected that a fight between Germany and the USSR would weaken the mentioned nations. As a result, Stalin decided to create a geographical barrier or buffer in Eastern Europe in case of a future agression by the Western nations. This barrier included the nations that had been invaded by the Red Army on their way to Berlin and included East Germany and were included in the Warsaw Pact of 1955.